1. Field of the Invention
The invention relates to a pre-amplification circuit, and more particularly to a pre-amplification circuit preferably used as a front stage to an apparatus for receiving light.
2. Description of the Related Art
There is an urgent need for construction of a system for transmitting an optical signal at a super-high speed, in order to absorb rapid increase in communication volume accompanied with popularization of Internet and/or multi-media communication. A pre-amplification circuit constituting a receiver front end module which is one of components required to have a high operation rate in the above-mentioned system is necessary to have a preferred gain-frequency characteristic and a preferred output characteristic over a broad range. As such a pre-amplification circuit, there has been conventionally used a trans-impedance type pre-amplification circuit.
When a pre-amplification circuit is used as a receiver module, a photodetector is connected to an input terminal of the pre-amplification through a bonding wire, and an electric signal is input into the pre-amplification circuit in accordance with light entering the photodetector. Each of amplifier stages constituting the pre-amplification circuit is electrically connected to a power source through a bonding wire, a bypass capacitor and a lead wire, and direct-current bias current is applied to each of the amplifier stages.
The conventional receiver module is accompanied with two problems.
The first problem is a peak found in a gain-frequency characteristic.
In an optical receiver module receiving a data signal having a frequency of a GHz order, inductance (hereinafter, referred to as “L”) and capacitance (hereinafter, referred to as “C”) of wirings and bonding wires in circuits are not ignorable. Hence, an excess gain-peak is found in a certain frequency range in a trans-impedance gain-frequency characteristic. This gain peak deteriorates flatness of the characteristic.
The second problem is deformation in output waveform, caused when the output waveform is saturated in receiving much light.
Light entering a photodetector increases or decreases in accordance with a transmission distance in a light-transmitting system, and accordingly, an amplitude of a current signal to be input into a pre-amplification circuit increases or decreases. Hence, a pre-amplification circuit is required to cover a wide range of amplitude of an input current, in order to cover a wide range of transmission distance.
However, in general, as an amplitude of an input current increases beyond a base amplitude, a waveform of an output signal transmitted from a pre-amplification circuit is saturated and thus deformed. Finally, it becomes impossible for the waveform to respond to an input current without error.
This is because when an amplitude of an input current increases beyond a base amplitude, a current is introduced too much into an input section of a pre-amplification circuit, and hence, a voltage at an input section of an pre-amplification circuit is raised.
One of solutions to the first problem is to arrange a capacitor between an input section of a pre-amplification circuit and a grounded voltage.
A capacitance of a newly added capacitor would reduce influence exerted by inductance and/or capacitance of wirings and bonding wires in circuits in a module, and hence, could prevent a gain peak.
As a solution to the second problem, Japanese Patent Application Publication No. 4-225630 has suggested an optic receiver including an optic receiver and a Schottky barrier diode electrically connected in series to each other. A node through which the optic receiver is electrically connected to the Schottky barrier diode is electrically connected to an input terminal of a low-noise amplifier through a resistance and a capacitor electrically connected in series to each other.
In the suggested optic receiver, when a level of received light is low, and thus, a light-receiving element transmits a small current, the Schottky barrier diode is off, and accordingly, a current output from the light-receiving element is almost wholly introduced into the low-noise amplifier. On the other hand, when a level of received light is high, and thus, the light-receiving element transmits a much current, the Schottky barrier diode becomes on, and hence, only a limited current is introduced into the low-noise amplifier.
However, in the above-mentioned solution to the first problem, the capacitor arranged between an input section of an amplifier circuit and a grounded voltage does not reduce a current to be introduced into an input section of a pre-amplification circuit or a voltage to be applied to a pre-amplification circuit, and hence, cannot prevent deformation in a waveform of a signal output from a pre-amplification circuit which deformation is caused when a current having an amplitude exceeding a base amplitude is introduced into a pre-amplification circuit. After all, the deformation is identical with deformation observed when a capacitor is not arranged between an input section of an amplifier circuit and a grounded voltage.
That is, the solution to the first problem cannot solve the above-mentioned second problem.
The optic receiver suggested in Japanese Patent Application Publication No. 4-225630 does not suggest a solution to the first problem, and hence, the problem that an excess gain-peak found in a trans-impedance gain-frequency characteristic deteriorates flatness of the characteristic remains unsolved.
Japanese Patent Application Publication No. 5-243858 has suggested a trans-impedance type optical amplifier including a feedback having a resistance Rx for determining a gain, and an output transistor. An input current Iin and an output voltage Vout have a relation defined as follows.Vout=−Iin×Rx
A diode is electrically connected in parallel to the feedback resistor in a forward direction, and means for varying a gain which means determines an input current at which the diode is turned on is electrically connected in series to the output transistor.
Japanese Patent Application Publication No. 10-242774 has suggested a light-receiving front-end amplifier including a feedback type trans-impedance circuit comprised of an amplifier including a transistor having a grounded emitter, and a feedback resistor, and further including a transistor having an emitter and a base both electrically connected in parallel to the feedback resistor, and a collector electrically connected to a reference voltage point.
Japanese Patent Application Publication No. 9-260960 has suggested an optic amplifier comprised of a photoelectric transfer device which is electrically connected to an input terminal and to which a bias voltage is applied, a trans-impedance type front-end amplifier to which an output current transmitted from the photoelectric transfer device is input, a circuit for controlling a gain of the trans-impedance type front-end amplifier, and a waveform-shaping circuit electrically connected to an output of the trans-impedance type front-end amplifier.
In view of the above-mentioned problems in the prior art, it is an object of the present invention to provide a pre-amplification circuit which is capable of solving the above-mentioned first and second problems, having a trans-impedance gain-frequency characteristic which is flat over a wide range of frequency, and suppressing deformation of a waveform of a signal transmitted therefrom even when an excess-current signal is introduced thereinto.